The Friend virus genome: Evidence for the stable association of MuLV sequences and sequences involved in erythroleukemic transformation

Abstract

Studies on the genetics and molecular biology of the Friend virus complex, which includes both a spleen focus-forming virus (SFFV) and a lymphoid leukemia helper virus (LLV), have been hampered by the apparent inability to propagate SFFV in vitro under clonal conditions. The present study describes the establishment of an NIH/3T3 mouse fibroblast culture which continuously releases high titers of both LLV and SFFV into the culture medium. SFFV harvested from such cultures was in excess of its LLV helper virus and titrated in vivo with multi-hit kinetics. Hybridization experiments, using purified 70S viral RNA and cDNA made from Friend virus stocks containing SFFV in excess of its helper LLV, indicated that approximately 25–30% of this cDNA represents SFFV-specific sequences. By use of this virus stock, several mouse and rat clones nonproductively infected with SFFV were isolated. SFFV rescued from these nonproductively infected clones by superinfection with LLV, as well as SFFV produced by chronically infected NIH/3T3 cells, was subject to restriction by a previously described host regulatory gene, Fv-2. Each of several SFFV nonproducer clones was shown to contain relatively large amounts of viral-specific RNA sequences. Moreover, these clones also expressed high levels of a 15,000 molecular weight virion structural protein, p15, while the levels of the other gag gene-coded proteins and the major viral envelope glycoprotein, gp70, were similar to those exhibited by uninfected cells. The stable association between the erythroleukemic activity of SFFV and the gag gene-coded protein, p15, of murine leukemia virus is discussed in terms of a possible model for the generation of the SFFV genome.